A variety of human diseases are caused by genetic aberrations. Such aberrations may be due to gene mutations or chromosomal abnormalities. The application of recombinant DNA technology to the study of genetic diseases has greatly increased the knowledge regarding their molecular basis. As a result, the accuracy of diagnosis of a number of genetic diseases has substantially improved in recent years.
In addition, prenatal diagnosis of certain chromosomal abnormalities has become a routine procedure in clinical medicine. Such diagnosis is usually performed in cases where the parental age is relatively advanced, or there is a family history of inherited diseases. For instance, it has been recommended that pregnant women over the age of 35 undergo prenatal diagnosis. At present, the diagnosis is carried out by a procedure known as amniocentesis which involves the aspiration of a small sample of amniotic fluid from the pregnant mother, culturing the fetal cells in the fluid, and determining the karyotype of the fetal cells. More recently, chorionic villus sampling has also been used, which involves the direct transcervical and transabdominal aspiration of the chorionic villus. However, since both amniocentesis and chorionic villus sampling require invasive procedures for obtaining fetal cells, they inevitably expose both the mother and the fetus to a certain amount of risk. Therefore, non-invasive approaches to prenatal diagnosis are preferred.
It has now been established that a small number of fetal cells circulate in maternal blood, which provide an alternative and desirable source of materials for prenatal genetic testing (Simpson and Elias, 1993, JAMA 270:2357). However, in order to successfully utilize maternal blood for prenatal diagnosis, it is recognized that the small number of fetal cells must first be enriched, and one must employ highly sensitive and specific techniques to detect the fetal cells (Holzgreve et al., 1992, J. Reprod. Med. 37:410). While several detection methods have been made available through recent advances, including polymerase chain reaction (PCR) and fluorescence in situ hydribization (FISH), the major difficulty in the routine use of maternal blood for prenatal diagnosis is the inability to enrich the small number of fetal cells in a mixture of maternal cells to yield reliable diagnostic results.
One of the ideal fetal cell populations in maternal blood is nucleated red blood cells. Although there have been wide variations in the estimation of the ratio of fetal nucleated red blood cells to maternal red blood cells, one recent report estimates approximately one fetal cell to 1.times.10.sup.7 -1.times.10.sup.8 maternal cells (Simpson and Elias, 1993, JAMA 270:2357). Thus, due to the extreme rarity of fetal cells in maternal blood, a number of cell separation schemes have been designed to enrich fetal cells prior to genetic testing, including the use of fluorescence-activated cell sorting (Herzenberg et al., 1979, Proc. Natl. Aca. Sci. USA 76:1453), magnetic-activated cell sorting (Ganshirt-Ahlert et al., 1992, Am. J. Obstet. Gynecol. 166:1350) or a combination of these procedures (Ganshirt-Ahlert et al., 1992, Am. J. Hum. Genet. 51:A48). While these procedures have been able to partially enrich fetal cells, they are both costly because of the need for sophisticated instrumentation, and cumbersome due to multiple steps. More importantly, the currently used enrichment methods all result in substantial cell loss, thereby reducing the number of fetal cells for subsequent analysis. Thus, the low number of fetal cells in maternal blood has precluded their use in routine prenatal testing. At present, there remains a need for a rapid and reproducible procedure suitable for processing a large volume of whole blood, and which produces high-yield enrichment of fetal cells from maternal blood.